Download Interfacing Telephony Signals to SigLab

APPLICATION NOTE
Interfacing Telephony Signals to SigLab™
It is often necessary to perform measurements on analog telephone lines. The SigLab®
differential inputs make it easy to interface to these lines. This application note describes one
approach to constructing this interface.
Line to Input / Output
Coupling
Overview
Typical Telephone Signal
Characteristics
A Differential Voltage Divider
Analog telephone lines have a wide range of
signal levels. During the ring period, signals
of 130 V peak are present. Voice levels are
on the order of a few hundred mV peak-topeak riding on approximately 10 V of DC
offset. The telephone line is balanced with a
nominal 600 ohm impedance, therefore
requiring differential voltage measurements.
Given the signal levels and impedances
previously mentioned, a passive voltage
divider will do the job. Replacing the two
standard resistor configurations (assuming a
2-channel interface) with the configuration
shown in Figure 2 provides an 11:1
attenuation.
Input BNC
SigLab Signal Conditioning Features
The standard SigLab input full-scale range
is ±10 V. An access hatch in SigLab's top
cover allows the user to install custom
signal conditioning circuitry.
Opening the access hatch exposes two
14-pin dual-in-line headers (one for each
channel) with three resistors connected as
shown in Figure 1. These headers are behind
the input BNC connectors. SigLab's input is
a differential type with a 1 Mohm
differential input impedance.
Input BNC
1
130
2
130
14
13
To subsequent
stages
+
Rin=1M
-
499
12
System Gnd.
(output BNC
shell)
Resistor Values in ohms
Figure 1 - Standard Input Conditioning
1
2
10K
14
13
+
Rin=1M
-
3
1K
1K
System
Gnd.
Figure 2 - Telephony Input Conditioning
Several points should be noted:
1) The 11:1 scale factor may seem odd (10:1
more natural), but the 11:1 scaling is
handled easily by way of the engineering
units in the VI software. This attenuation
provides a full-scale capability of ±110 V.
The system actually works fine to the 130 V
levels encountered on the line due to the
extra margin built into SigLab's front end.
2) The 22 Kohm differential input
impedance is more than sufficient for
telephony work given the nominal 600 ohm
line impedance.
3) The accuracy and common mode
rejection are a function of the resistor
Application Note 2.0 A Interfacing Telephony Signals to SigLab
1/00 SLAP2
10K
1
tolerances. For best results, use 1% or better
resistors.
displayed data. These factors are also stored
with the measurement data in the
appropriate file.
A Transformer for the Output
Providing a signal to the telephone line is
slightly more complicated. Normally the
system (PC and SigLab) ground path is via
the SCSI cable to the host computer and
then to "ground" via the usual safety ground
conductor in the power cord. One solution is
to eliminate this ground path by running a
notebook PC and SigLab from battery
power. A better method is to use a telephone
coupling transformer with a 1:1 turns ratio.
The objective is to provide a stimulus to the
telephone line for a transfer function
measurement. The precise characteristics of
the transformer (turns ratio, loss, frequency
response, etc.) are not critical since the
stimulus will be monitored after the
transformer.
Figure 3 shows the connections to the
coupling transformer along with a 560 ohm
series resistor providing an AC source
impedance of approximately 600 ohms. A
fine choice for the transformer, among many
others, is the Stancor type TTPC-1, -2, -6, or
-7 (available from Newark Electronics).
50 Ohms
1:1
560 Ohms
0
SigLab
External
On/ Off-Hook
If the impedance presented to the line is
high (e.g. 22K ohms) the line does not "see"
this loading and behaves as though the line
is inactive (the telephone is on the hook).
This is useful for making non-intrusive
measurements.
If an impedance of 600 ohms is connected to
the line, the line goes to the off-hook state
and behaves as though the telephone
handset has been lifted from the cradle. A
dial tone signal is created when the line goes
from the on-hook to off-hook state.
A Measurement Example
Figure 4 shows a typical electrical
measurement configuration for a 2 channel
measurement, including coupling a source
to the line. SigLab's 2 input channels are
assumed to be configured as in Figure 2.
This configuration allows both single
channel measurements (power spectrum or
time-domain), as well as a 2 channel
transfer function measurement.
Two telephone lines are connected to
SigLab input channels, and Line 1 is
coupled to the output generator. A standard
telephone is also connected to the lines. A
single-line phone will suffice, but with a
two line phone transfer function
measurements can be made for calls in both
directions.
Figure 3 - Output Interface
Making Measurements
Engineering Units for Scaling
The vos, vsa, vna and vid virtual instruments support engineering units. Using the
engineering units allows the proper scale
factors for the input to be applied to the
2
Application Note 2.0 A Interfacing Telephony Signals to SigLab
1/00 SLAP2
SigLab
OK
L1
Model
20-22
Power
On
L2
1:1
Standard 2 line
telephone
1 2 3
4 5 6
7 8 9
*
0 #
S1
560 ohms
Outgoing
Line #1
V1
620 ohms
S2
Incoming
Line #2
V2
Figure 4 - Two Channel Measurement
The following steps are taken to set up for
the transfer function measurement:
1. Open S1 and S2.
2. Select line 1 with the telephone.
3. Pick up the handset, and dial line 2's
number.
4. When line 2 rings, close S2.
5. Close S1, and hang up the handset.
Telephone lines 1 & 2 are now connected
through the local switching office. The
transfer function of the circuit (through the
telephone office) can be measured now.
1. vna: network analyzer (FFT based).
2. vss: network analyzer (swept sine).
3. vid: system identification.
The signal to noise ratio and linearity of the
telephone system make the vna a good
choice for this measurement.
Figure 5 shows the setup and resulting
frequency response measurement of the
vna. For a power spectral or a time domain
measurement we would need to turn
engineering units on to account for the
attenuators. However for this transfer
function the engineering units are not
needed since the attenuation is the same on
both channels.
Frequency Response
An interesting measurement is the frequency
response of a station-to-station call.
SigLab has three different virtual
instruments for measuring the frequency
response of a linear system:
A chirp excitation serves as the stimulus to
the system. The chirp contains energy
throughout the selected analysis band of DC
to 5 kHz. It is injected into line 1 via the
transformer. Note that channel 1 is
connected directly across line 1 to monitor
the excitation. The frequency response ofthe
transformer therefore does not affect the
Application Note 2.0 A Interfacing Telephony Signals to SigLab
1/00 SLAP2
3
measurement. The measurement provides
the relationship of V2 to V1.
The measurement reveals a 16 dB loss at
approximately 300 Hz and a roll-off of 12
dB more at 3 kHz. This leads to an ultimate
attenuation of 28 dB at the 3 kHz band edge.
Conclusion
This note describes how to interface
SigLab's inputs and outputs to standard
analog telephone lines. As a measurement
example, the vna (network analyzer) is used
to characterize station to station
transmission properties. Many other
measurements are possible with the SigLab /
MATLAB combination including:
• Group delay.
• Line modeling (differential equations).
• Line impedance.
• Noise and distortion.
• Cross talk.
• Transient response.
• Long record excitation and response.
These measurements provide a means to
fully characterize a typical telephone
connection.
Figure 5 - Frequency Response
Figure 6 shows both the magnitude and
phase of the line transfer function plotted
with a log x-axis (Bode plot). The plot was
captured using the Windows Metafile
format in the vna and then imported directly
into this document. The Metafile format
provides the best graph quality.
Bode Plot
For more information contact:
Spectral Dynamics
1010 Timothy Drive
San Jose, CA 95133-1042
Phone: (408) 918-2577
Fax: (408) 918-2580
Email: [email protected]
www.spectraldynamics.com
600
400
200
Magnitude
0
-200
-400
Phase
-600
-800
Figure 6 - Telephone Line Bode Plot
© 1995-2002 Spectral Dynamics, Inc.
SigLab is a trademark of Spectral Dynamics, Inc. MATLAB is a registered trademark and Handle Graphics is a trademark of The
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in U.S.A.